1. Field of the Invention
The invention generally relates to the art of measuring gaseous substances and, more specifically, to method and apparatus means for determining specific thermal conductivity parameters of a gaseous substance of analytic interest termed "target gas" herein. The term "gaseous" is intended to indicate that the substance of interest is gaseous under normal operating conditions of the subject method, e.g. at temperatures in the range of from -10.degree. to 200.degree. C. at pressures ranging from ambient or reduced pressures to 10 bars or more. Typically, the target gas will be contained in a fluid medium; the term "fluid" is used herein to encompass both normally liquid (i.e. liquid under normal ambient conditions) as well as normally gaseous media. Typically, the specific thermal conductivity parameters that are measured according to the invention will not be of primary interest as such but will be used to determine derived parameters of the target gas, e.g. its concentration in a given medium or its pressure.
2. Description of the Prior Art
Numerous methods for monitoring gas concentrations are known and are frequently classified according to the measuring principles involved and/or the characteristic means used for separating the substance of interest from a mixture.
Accordingly, when using a membrane that is permeable for the substance of interest but impermeable to other components of the mixture, the term "membrane-based" analysis is appropriate. When membranes are used mainly because of their separating effect, e.g. between a gas, such as carbon dioxide, and a liquid, such as a carbonated beverage, maximum permeability of the membrane for the gas of interest is generally assumed to be desirable.
For example, a prior art membrane-based method of determining the concentration of carbon dioxide in a beverage, such as beer, as disclosed in EP 0 043 229 and EP 0 103 988, uses membranes that exhibit a maximum permeability for the gas of interest because the actual measuring parameter in these methods is the so-called equilibrium pressure that will build up within a membrane-enclosed chamber in response to the "concentration" (synonymous herein with "partial pressure") of the gas in a liquid that is "external" to the membrane-enclosed space. However, even with membranes made of silicone rubber which are notorious for their high permeability to gases, such as carbon dioxide, relatively long periods of time of typically in the order of 20 minutes may be required until equilibrium of gas pressures.